Damp Air

Science
Science brain teasers require understanding of the physical or biological world and the laws that govern it.

A cubic foot of dry air at standard pressure and temperature is weighed on a scale. Then a cubic foot of damp air at the same pressure and temperature is weighed. Which will weigh more?

Hint

Think about molecular weights.

Answer

The damp air weighs less because water molecules (molecular weight of 18) have replaced more massive molecules of air (nitrogen: molecular weight of 28, oxygen: molecular weight of 32). Both volumes have the same number of molecules. Therefore, a humidity increase lowers the barometer reading. The barometer pressure value gets lower as a storm approaches. (This question is from "Mad About Physics", a must have book for all science types.)